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The autonomic nervous system, which includes the sympathetic and the parasympathetic systems, governs involuntary actions of cardiac muscle and certain tissues of the human body. Autonomic nervous system neuropathy, which is sometimes referred to as visceral neuropathy, affects the nerves that serve the heart and internal organs and produces changes in many processes and systems. Symptoms of autonomic neuropathy include numbness and pain in the hands, feet, or legs, digestive dysfunction, sexual dysfunction, and orthostatic hypotension, disordered bowel, bladder or abnormal papillary reflexes.
Autonomic neuropathy is most commonly linked to diabetes, however, several causes are possible, including alcoholism, sleep apnea, and coronary artery disease. Thus, evaluation of the autonomic nervous system function has wide applicability, from diagnosing and treating patients with diabetes to detecting patients at risk for sudden death due to cardiac arrest.
One technique for detecting autonomic neuropathy is to analyze the variability of a patient""s heart rate. Various tests have been used to exercise the autonomic nervous system for this purpose, including the Valsalva test and the Expiration/Inspiration (E/I) test which is sometimes referred to as the metronomic deep breathing test. The Valsalva test requires that the patient forcibly exhale to a predetermined pressure, such as 40 mmHg, for a predetermined duration, such as 15 seconds, during which the heart rate is monitored. Conventionally, the heart rate is measured by measuring the interval between certain phenomena of the patient""s ECG signal, such as the interval between peaks of the QRS complex, or the R-R interval. Thereafter, the patient rests for a predetermined duration. The result of the Valsalva test is a ratio of the highest heart rate (as indicated by the shortest R-R interval) during the breathing maneuver to the lowest heart rate (as indicated by the longest R-R interval) during a recovery period after the maneuver. In accordance with the E/I test, the patient is instructed to breathe deeply at a frequency of 6 cycles/minute, which has been shown to produce maximal heart rate variability in healthy individuals. The result of the E/I test is a ratio of the average of the heart rate peaks to the average of the heart rate troughs.
Several additional tests of heart rate variability are described in one or more of the following papers: (1) D. Ziegler, et al. entitled xe2x80x9cAssessment of Cardiovascular Autonomic Function: Age-related Normal Ranges and Reproducibility of Spectral Analysis, Vector Analysis, and Standard Tests of Heart Rate Variation and Blood Pressure Responses.xe2x80x9d Diabetic Medicine, Vol. 9, pgs. 166-175, 1992; (2) I. O""brien et al. entitled xe2x80x9cHeart Rate Variability in Healthy Subjects: Effect of Age and the Derivation of Normal Ranges for Test of Autonomic Function,xe2x80x9d Br Heart J, 1986, 55:348-54; (3) D. Ewing, entitled xe2x80x9cCardiac Autonomic Neuropathy,xe2x80x9d Chapter 5, Jarret (ed) Diabetes and heart Disease, 1984 Elesvier Science Publishers B.V.; and (4) B. Clarke, et al. entitled xe2x80x9cCardiovascular Reflex Tests,xe2x80x9d New York State Journal of Medicine, May 1982, pages 903-908. Further heart rate variability tests including high frequency area under the Power Spectrum Density (PSD), low frequency area under the PSD, and very low frequency area under the PSD are described by J. Saul in xe2x80x9cBeat-to-Beat Variations of Heart Rate Reflect Modulation of Cardiac Autonomic Outflow,xe2x80x9d NIPS, Volume 5: 32-7, February 1990.
Typically, the physician is provided with the individual test results and standard data against which to assess the results. The accuracy of heart rate variability evaluation as a measure of autonomic nervous system function is dependent on many factors, including the precision with which the patient follows the prescribed breathing regimen which affects the intra-patient reproducibility of the tests, the inter-patient reproducibility of the tests, and the accuracy with which the R-R intervals of the ECG signal are detected. Many improvements to the accuracy of heart rate variability evaluation have been made. For example, a system for enhancing a patient""s compliance with a predetermined breathing regimen is described in U.S. patent application Ser. No. 08/942,710 entitled xe2x80x9cMETHOD AND APPARATUS FOR ENHANCING PATIENT COMPLIANCE DURING INSPIRATION MEASUREMENTS.xe2x80x9d Further, methods and apparatus for enhancing the accuracy of detection of R-R intervals are described in U.S. Pat. No. 5,984,954 entitled xe2x80x9cMETHODS AND APPARATUS FOR R-WAVE DETECTION.xe2x80x9d
It is an object of the present invention to provide an accurate indicator of a patient""s autonomic nervous system function.
Another object of the invention is to provide a single value measure of a patient""s autonomic nervous system function.
These and other objects of the invention are achieved by combining the results of different autonomic nervous system tests, each referenced to a cross-sectional population, to provide a single autonomic function indicator. A method according to the invention includes the steps of generating a plurality of mathematical expressions, each associated with a different autonomic nervous system test and being a function of physiological data from the cross-sectional population. The method further includes measuring physiological data of the patient and inserting the measured physiological data into each of the mathematical expressions to provide an output value for each of the mathematical expressions. The output values of each of the mathematical expressions are combined to provide the indicator of the patient""s autonomic nervous system function.
The above-described method and the resulting autonomic function indicator provide a highly accurate assessment of a patient""s autonomic nervous system function for several reasons. First, since the autonomic function indicator of the present invention is a composite of a plurality of autonomic nervous system measurements, errors associated with the performance and evaluation of individual tests are minimized, thereby enhancing both intra-patient and inter-patient reproducibility. Also, there is greater contrast between the autonomic function indicator of healthy individuals and those with autonomic neuropathy, as compared to conventional autonomic nervous system test results, thereby facilitating accurate interpretation of the indicator. Further, the autonomic function indicator of the present invention is provided in a standard form suitable for standardized interpretation (e.g., a value between 0 and 1) regardless of the number of different tests which are performed and used to provide the indicator. The autonomic nervous system function indicator provides a convenient tool for long-term tracking of a patient""s autonomic nervous system function, and the efficacy of intervention.
In an illustrative embodiment, each mathematical expression is generated by sorting physiological data of the cross-sectional population, ranking the sorted data, normalizing the ranked data, plotting the sorted data with respect to the normalized data, interpolating the plotted data, and representing the interpolated data as the mathematical expression. Preferably, the interpolation is a linear interpolation and the resulting mathematical expression is a line function.
In accordance with a further aspect of the invention, the mathematical expression generated for each autonomic nervous system test is a function of the age of the patient. With this arrangement, the natural degradation of the autonomic nervous system which occurs with age does not affect the indicator.